neuroml: do not use "is" with strings

This works, but only because of an implementation detail: short
strings are interned as part of module code. But the rules are complicated
and we should not rely on this.

See http://guilload.com/python-string-interning/ for a nice writeup.

Python3.8 now warns about this.

Author: keszybz

python/moose/neuroml/MorphML.py

@@ -585,7 +585,7 @@ def set_compartment_param(self, compartment, name, value, mechanismname):
             compartment.refractoryPeriod = value # compartment is a moose.LIF instance (intfire)
         elif name == 'g_refrac':
             _logger.info("SORRY, current moose.LIF doesn't support g_refrac.")
-        elif mechanismname is 'synapse': # synapse being added to the compartment
+        elif mechanismname == 'synapse': # synapse being added to the compartment
             ## these are potential locations, we do not actually make synapses,
             ## unless the user has explicitly asked for it
             if self.createPotentialSynapses:
@@ -595,7 +595,7 @@ def set_compartment_param(self, compartment, name, value, mechanismname):
             ## I assume below that compartment name has _segid at its end
             segid = compartment.name.split('_')[-1] # get segment id from compartment name
             self.segDict[segid][5].append(value)
-        elif mechanismname is 'spikegen': # spikegen being added to the compartment
+        elif mechanismname == 'spikegen': # spikegen being added to the compartment
             ## these are potential locations, we do not actually make the spikegens.
             ## spikegens for different synapses can have different thresholds,
             ## hence include synapse_type in its name